GRTU has sponsored an ongoing research project at Southwest Texas State University for approximately three years. The purpose of the study is to examine the Guadalupe River as a trout habitat. Some of the activities in the project include sampling insect populations and the preferences of the trout for eating insect types. Project goals include developing stocking strategies and investigating methods for enhancing trout habitat.

At the January GRTU meeting, Thorpe Halloran delivered a research update on the project. A summary of the insect sampling study appears in this newsletter. Many were surprised by the preponderance of scuds and midges, which accounted for over 50 percent of the insects in the drift in the April 1998 sampling.

Here are some other facts that come from the report, some generated by our research and others from literature searches done by SWSTU on tailwater fisheries:

Stocked rainbow trout begin to identify and feed on stream food items just four days after being release.

Reactive distance (distance from which a trout initiates its approach to prey) doubles in less than week–this means that the fish get twice as smart about food just a week after stocking!

91 percent of the trout taken for sampling from the Guadalupe had full stomachs–they are clearly finding enough food.

Examination of food in trout stomachs indicates that bottom-dwelling insects make up a substantial part of the trout diet–and you wondered why nymphing is so effective!

One aspect of the study is to determine the dietary preferences of Guadalupe trout. Initial results from the April 1998 sampling show the preferentially feed on baetis mayflies (blue winged olives), midges, and drifting caddis.

These tidbits, and the data in the insect drift sampling studies, are just part of the return on GRTU's investment that the SWSTU project has produced. This is the final year of the study, and additional results will be distributed to GRTU members as they become available.

Alan Bray, Director, Ex-Officio

April 1998 Drift Samples from SWSTU Study

Noon-3 PM

3 - 6 PM

6 - 9 PM

Average

Scuds

43%

14%

22%

26%

Spiders, Beetles, Bees, Ants, Moths

4%

11%

5%

6%

Midges

Midge (Chironomidae)

0%

23%

15%

13%

Black Flies (Simuliums)

21%

15%

24%

20%

Total Midges

21%

38%

40%

33%

Crayfish

1%

0%

0%

0%

Mayflies

Blue Wing Olive (Baetis)

5%

9%

9%

8%

Tricos (Tricorythidae)

6%

6%

15%

9%

Caenis

0

0

1%

0%

Heptagenia

0

0

1%

0%

Isonychia

0

0

1%

0%

Total Mayflies

11%

15%

26%

17%

Snails

1%

0

0

0%

Dragon flies

1%

0

0

0%

Caddisflies

Speckled & Spotted Sedge (Helicopsyche)

7%

14%

7%

9%

Small Spotted Sedge (Hydropsyche)

3%

0

0

1%

Microcaddis (Hydroptilidae)

2%

0

0

1%

Long Horn Sedge (Leptoceridae)

1%

6%

0

2%

Brown Checkered Summer Sedge

1%

3%

1%

2%

(polycentropodidae)

Total Caddisflies

13%

23%

8%

15%

Flat worms

5%

2%

1%

3%

Notes:

Scuds+Midges+Mayflies+Caddisflies = 91% of drift

Mayflies and Midges are most common in the evening drift

Caddisfly pupae and larvae are most common in mid-afternoon drift

Scuds are most common in the early drift, but are available throughout the day